1. Field of the Invention
The present invention relates to a fuel cell stack comprising a stacking body formed by stacking a plurality of assemblies each interposed between separators. Each of the assemblies includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. The stacking body is tightened by a pair of end plates provided at opposite ends of the stacking body in the stacking direction.
2. Description of the Related Art
In recent years, various types of fuel cells are developed. For example, a solid polymer electrolyte fuel cell is known. The solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon sheet. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell (unit cell) for generating electricity. A plurality of unit cells are connected together to form a fuel cell stack.
In the fuel cell of the fuel cell stack, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
The fuel cell stack is attractive for use in vehicles. For example, FIG. 13 shows a fuel cell stack 10 mounted on a vehicle body 1 of a vehicle such as an automobile. In the fuel cell stack 10, a plurality of unit cells 12 are electrically connected in series. In FIG. 13, the unit cells 12 are stacked laterally to form a stacking body 13.
Each of the unit cells 12 includes a pair of separators 22a, 22b and a membrane electrode assembly 20 interposed between the separators 22a, 22b. The membrane electrode assembly 20a includes an anode 14, a cathode 16, and an ion exchange membrane (electrolyte layer) 18, interposed between the anode 14 and the cathode 16. Each of the separators 22a, 22b has a first gas flow passage 24 on its surface facing the anode 14, and has a second gas flow passage 26 on its surface facing the cathode 16. A fuel gas such as a hydrogen-containing gas flows along the anode 14 through the first gas flow passage 24, and an oxygen-containing gas such as air flows along the cathode 16 through the second gas flow passage 26.
Terminal plates 34a, 34b are electrically connected to the outermost unit cells 12 disposed at opposite ends of the stacking body 13 in the stacking direction, respectively. Insulator plates 36a, 36b for prevention of electric leakage are stacked on the outside of the terminal plates 34a, 34b, respectively. End plates 38a, 38b are stacked on the outside of insulator plates 36a, 36b, respectively. Further, back up plates 40a, 40b are disposed outside the end plates 38a, 38b, respectively. The unit cells 12, the terminal plate 34a, 34b, the insulator plates 36a, 36b, end plates 38a, 38b, and back up plates 40a, 40b make up the fuel cell stack 10.
A plurality of spring members 42 such as belleville-springs are interposed between the end plate 38a and the back up plate 40a for maintaining electrical connections between the adjacent unit cells 12.
In the peripheral area of the fuel cell stack 10, a plurality of through holes 44 are formed. The through holes 44 extend from the back up plate 40a to the other back up plate 40b. Tie rods 46 are inserted into the through holes 44, respectively. Nuts 48 are threaded over the tie rods 46 to tighten the back up plates 40a, 40b. Therefore, the stacking body 13, the terminal plate 34a, 34b, and the end plates 38a, 38b are pressed together, and the belleville-springs 42 are compressed.
The fuel cell stack 10 is mounted on a vehicle body 1 by brackets 50, 52. The brackets 50, 52 are connected to the end plate 38a, the back up plate 40b, respectively. The bracket 52 is fixed to the vehicle body 1 by a bolt 54. The bracket 50 is slidable on the vehicle body 1.
An arm 56 extends from a lower end of the bracket 50. An oblong groove 60 having a step 58 is formed in the arm 56. A bolt 62 is inserted in the oblong groove 60. The step 58 is pressed by a head of the bolt 52 with a suitable force. In this manner, the bracket 50 is slidably mounted on the vehicle body 1.
When the stacking body 13 is thermally expanded in the stacking direction during the operation of the fuel cell stack 10, the belleville-springs 42 are compressed to some extent corresponding to the amount of thermal expansion. When the operation of the fuel cell stack 10 is stopped, and the temperature of the fuel cell stack 10 is lowered, the stacking body 13 is thermally contracted. Therefore, the belleville-springs 42 are stretched. The thermal expansion or contraction of the stacking body 13 causes the belleville-springs 42 to be compressed or stretched. Therefore, the tightening force applied to the stacking body 13 is maintained substantially.
The electrolyte layer 18 absorbs and releases water produced in the electrochemical reaction. Further, the electrolyte layer 18 absorbs and releases moisture in the fuel gas and the oxygen-containing gas. Therefore, the electrolyte layer 18 swells and shrinks in the stacking direction of the stacking body 13. Further, the membrane electrode assembly 20 wears out with the repeated use of the fuel cell stack 10. Therefore, the rigidity of the membrane electrode assembly 20 is reduced, and the size of the membrane electrode assembly 20 is reduced slightly. The size reduction also occurs in the sealing member for supporting the membrane electrode assembly 20, and the separators 22a, 22b. 
In the fuel cell stack 10, when the dimension of the components such as the electrolyte layer 18, the sealing member, the separators 22a, 22b changes in the stacking direction, the belleville-springs 42 are compressed or stretched correspondingly. Therefore, the stacking body 13 is constantly pressed together under a desirable pressure.
When the dimension of the components changes in the fuel cell unit 10, and the belleville-springs 42 are compressed or stretched, the bracket 50 guided by the oblong groove 60 and the bolt 62 slides on the vehicle body 1 in the stacking direction.
In the fuel cell stack 10, the bracket 50 is slidably mounted on the vehicle body 1. If both of the brackets 50, 52 are fixed on the vehicle body 1, large heat stress is applied to the fuel cell stack 1 due to the thermal expansion of the stacking body 13. Therefore, the bracket 50 is not fixedly mounted the vehicle body 1.
Since the bracket 50 is slidable on the vehicle body 1, it is not possible to apply large pressure equally to each of the unit cells 12, and to reduce the electric resistance in each of the unit cells 12 and between the adjacent unit cells 12. Therefore, the end plates 38a, 38b need to be thick, for example, for reducing the electric resistance between the adjacent unit cells 12.
When the fuel cell stack 10 is mounted on the vehicle body 1, vibrations and impacts generated while the vehicle is moving are applied to the bracket 52 only. Therefore, the bracket 52 needs to be large. Since both of the end plates 38a, 38b are not firmly fixed, the positions of the end plates 38a, 38b may be displaced undesirably due to the vibrations of the vehicle body 1. Further, the vibrations of the vehicle body 1 may decrease the sealing pressure on the surfaces of the components in the fuel cells tack 10, and leakage of the reactant gas or the coolant may occur.